1. Field of the Invention
This invention concerns a slide valve type screw compressor.
2. Description of the Prior Art
As illustrated in FIGS. 1 and 2, a screw compressor is generally provided with a pair of male and female screw rotors 5 and 6 (hereinafter referred to simply as "rotors" for brevity) which are rotatable in a meshed state within a compression chamber 8 in main casing 7. One end face of the compression chamber 8 is partly cut away at a position corresponding to tooth grooves of the rotors 5 and 6 to provide an opening 10 (an axial port) in communication with a suction port 11 through a suction casing 7a. The other end face is similarly provided with an opening, although different in shape, for communication with a discharge port 12. Further, provided beneath the compression chamber 8, partly in overlapped relation therewith, is a columnar gap space in communication with the suction port 11, slidably receiving therein a slide valve 31 (of a length L.sub.1) in the axial direction of the rotors. The slide valve 31 is provided with curved surfaces each of an arcuate shape in section which constitute part of the inner wall surface of the compression chamber 8, and its forward movement is limited by a fixed valve 31a which is located in a forward position.
In this sort of screw compressor, a gas which is sucked in through the suction port 11 is closed off and compressed in the compression chamber 8 between the rotors 5 and 6 and the casing 7, and then sent toward the discharge port 12, while the slide valve 31 is retractable to open a radial port 13 of a variable area in the wall of the compression chamber 8 for communicating the compression chamber 8 with the suction port 11, permitting volumetric control through adjustment of the initial closing position of the rotors 5 and 6.
However, as shown in FIGS. 3 and 4, the conventional slide valve 31 has end face 33 on the side of the suction port formed by a flat surface which is disposed perpendicular to the sliding direction, so that is has been difficult to preclude an abrupt and discontinuous variation in the volume of a closed space (hereinafter referred to simply as "suction volume" for brevity) at an initial closing point even if the radial port 13 is opened little by little in the initial stage of a volumetric control.
Now, the above-mentioned discontinuous variations are explained more particularly with reference to FIGS. 5 and 6 which show at (a) to (f), respectively, sequential phases of the rotation of the rotors.
More specifically, FIG. 5 shows at (a) to (f) varying conditions at the end of the compression chamber on the side of the suction port 11 in relation with rotation of the rotors. As the operation proceeds from phase (a) to (f), the rotors 5 and 6 are rotated successively in the arrowed directions, gradually compressing a closed space 14 which is indicated by a hatched area. In this instance, the closed space 14 is the one which is formed when the aforementioned radial port 13 is in a closed state (so that the latter does not appear in FIG. 5), and, for simplification of explanation, there is shown a case where the width W (illustrated in FIG. 2) of a lower projection 18 which forms the opening 10 in the end face 17 or which closes the ends of the screw root ends of the rotors 5 and 6 is equal to the width w (illustrated in FIG. 3) of the curved surfaces 2 of the slide valve 31.
Shown at (a) to (f) of FIG. 6 are developed views of sections taken along line VI--VI of FIG. 5, which correspond to phases (a) to (f) of FIG. 5. As shown there, an end face 33 of the slide valve 31 is positioned on the side of the suction port 11 and outside the compression chamber 8, closing the radial port 13 with the curved surfaces 2. (Therefore, the radial port 13 does not appear in FIG. 6.) The hatched areas in FIG. 6 indicate a closed space 14 corresponding to the hatched areas in FIG. 5, which is gradually shifted upward from phase (a) to (f) of FIG. 6. On the other hand, the closed space 14 reaches an end face 19 on the discharge side and the end of the discharging side is closed while the end face 19 is rotated through a predetermined angle, so that the volume of the closed space 14 is reduced to compress the gas gradually from phase (a) to (f) of FIG. 6.
Referring to FIG. 7, the slide value 31 is shown in a position which is slightly moved from that of FIG. 6 with its end face 31 located a little closer to the discharging side (the upper side in the figure) than the end face 17 of the compression chamber 8, with the radial port 13 in a slightly opened state, illustrating variations of the closed space in this position from phase (a) to (f) corresponding to the phases shown in FIG. 6. In this case, a portion corresponding to the closed space 14 is in communication with the suction port 11 through the radial port 13 as indicated by a dotted area in phases (a) to (d) of FIG. 7, so that it is only in and after phase (e) that a closed space 15 is formed as indicated by an hatched area. Namely, the lowermost point M (a closing point) of a V-shaped hatched area, at which the male and female rotors 5 and 6 contact with each other, is gradually shifted inward across the end face 17 of the compression chamber 8 and it is only when the closing point M reaches the end face 33 of the slide valve 31 that a closed space 15 is formed.
Therefore, the suctioning volume corresponds to the closed space 14 in phase (a) in the position of FIG. 6 and corresponds to the closed space 15 in phase (e) in the position of FIG. 7. Thus, the suction volume is abruptly varied discontinuously or stepwise from the volume in phase (a) of FIG. 6 to the volume in phase (e) of FIG. 7 (the same as that of the closed space in phase (e) of FIG. 6) upon opening the radial port 13 only in a slight degree. Even if the radial port 13 is further minimized, the result is that the position of the lowermost point M comes nearer to the end face 17 but the closed space 15 is not yet formed in phase (d) of FIG. 7 and is formed also in phase (e) of the same figure, resulting likewise in a suction volume which is varied discontinuously from the state in phase (a) of FIG. 6.
If the radial port 13 is widened by shifting the slide valve 31 toward the discharge end, the position of the lowermost point M which represents the initial closing point is shifted upward to reduce the suction volume continuously.
As is clear from the foregoing description, the suction volume is varied as indicated by curve II of FIG. 8, in which the horizontal axis represents a distance l of displacement of the slide valve 31, namely, the distance between the end faces 17 and 33 in the particular embodiment shown, and the vertical axis represents the rate (%) of the suction volume at various distances l of displacement to the suction volume in the state shown in FIGS. 5 and 6 (a state in which the radial port 13 is closed.)
As seen therefrom, curve II consists of a vertical portion AB and an inclined portion BC. The point A represents a state in which the end faces 17 and 38 are located in the same plane (distance of displacement l=0) with the radial port 13 closed, the point B represents a state in which opening of the radial port 13 has just been initiated or when the distance in phase (e) of FIG. 7 is infinitesimal, and the point C represents a state in which the radially port 13 has been further continuously widened. Thus, upon opening the radial port 13, the curve II is varied discontinuously from point A to B.
On such a discontinuous variation, a compressing gas which is of a relatively large mass like air shows an inferior response to the variation due to a greater frictional resistance, so that an apparent suction volume is varied continuously in response to displacement of the slide valve 31 as indicated by curve III (broken line) in FIG. 8. Namely, actually the suction volume can be controlled from the maximum value by gradually shifting the slide valve 31.
However, in a case where a light gas like hydrogen and helium is employed as a compressing gas, the gas has a low frictional resistance and shows a quick response to the aforementioned discontinuous variation, so that the apparent suction volume is varied discontinuously as indicated by curve II. Consequently, it has been difficult for the conventional screw compressor to control the suction volume of a light gas continuously in the initial state of the control.